Expansion-molded articles of thermoplastic resin can be prepared by a process wherein a chemical blowing agent or a gas, such as chlorofluorocarbon, butane, pentane, carbon dioxide or nitrogen, is directly supplied to and dissolved in the resin. The gas is incorporated into the molten resin and then the resin is injected into a mold. The blowing agent causes the resin to expand in the mold to form a part.
Various parts may be formed using the above-stated injection/foam molding process. For example, many in-vehicle armrests are currently formed using the stated process. Although the stated process provides a quick, inexpensive, and efficient technique for the high production of parts, the current systems utilized to form the parts are limited in their ability to adjust the density and density profile of the formed parts.
The material density of the parts can correlate to the stiffness and feel of those parts. The material density of the parts can also correlate to or affect the conforming or compliant nature of the parts, as well as the rebound response or ability of the parts to return to their original states and shapes. As such, part design versatility is also limited.
Some techniques currently exist for reducing the density of an injection/foam molded part. The techniques include increasing the volume of the utilized mold by moving the core half of the mold away from the cavity half of the mold. Although this increases the volume in which the part may expand, thus reducing the density of the part, this is not always feasible for complex part shapes. Also, the stated technique is limited in providing isolated or variable adjustment capability of material part densities.
Thus, there exists a need for an improved injection/foam molding process that provides increased ability in controlling the density and density profile of parts produced therefrom.